1. Field of the Invention
This invention relates to a fluid bearing device for supporting a rotary shaft such as a spindle of a machine tool by means of pressurized fluid.
2. Prior Art of the Invention
Generally, a fluid bearing device is provided in a circumferential direction thereof with a plurality of rectangular fluid pockets each of which has a supply port opening at the middle portion thereof. A rotary shaft is supported by means of pressurized fluid supplied into the fluid pockets through the supply ports. In such a fluid bearing device having the fluid pockets of the aforementioned shape, the flowing mode of the fluid within the fluid pockets is changed from laminar flow to turbulent flow as the rotational speed of the rotary shaft increases, whereby the shearing stress is increased. This causes the fluid frictional force to increase in square functional relationship with increase in the rotational speed of the rotary shaft, as indicated by the dotted line (a) of the graph in FIG. 5. Since energy loss becomes large within a high-speed rotational range of the rotary shaft, there occurs a problem that a driving motor for the rotary shaft and a cooling facility for operating fluid must be of higher capabilities, thereby resulting in larger size.
Further, in the case where the port for supplying the pressurized fluid opens at the middle portion of each fluid pocket in the circumferential direction, the pressure in each fluid pocket is high at the downstream side in the rotational direction of the rotary shaft and low at the upstream side, during the high-speed rotations of the rotary shaft. Under the condition, the pressurized fluid flows from the downstream side back to the upstream side, that is in reverse to the rotational direction of the rotary shaft, whereby the energy loss is increased.